The present invention concerns a PEM fuel cell stack consisting of membrane electrode assemblies, gas distribution layers and bipolar plates stacked one above the other. The invention particularly concerns PEM fuel cell stacks containing gas distribution layers consisting of woven carbon fibers.
Fuel cells convert a fuel and an oxidizing agent, which are locally separated from one another at two electrodes, into electricity, heat and water. Hydrogen or a hydrogen-rich gas can be used as the fuel, oxygen or air as the oxidizing agent. The process of energy conversion in the fuel cell is characterized by a particularly high efficiency. For this reason fuel cells in combination with electric motors are gaining growing importance as an alternative to conventional combustion engines.
The compact design, power density and high efficiency of so-called polymer electrolyte fuel cells (PEM fuel cells) make them suitable for use as energy converters in electric cars.
In the context of this invention the term PEM fuel cell stack refers to a stack of fuel cell units. Fuel cell unit is also abbreviated to fuel cell below. Each unit contains a membrane electrode assembly positioned between bipolar plates, also known as separator plates, which serve to supply gas and conduct electricity. A membrane electrode assembly (MEA) consists of a polymer electrolyte membrane, both sides of which are provided with reaction layers, the electrodes. One of the reaction layers takes the form of an anode for oxidizing hydrogen and the second reaction layer that of a cathode for reducing oxygen. So-called gas distribution layers made from carbon fiber paper or carbon fiber fabric or cloth, which allow good access by the reaction gases to the electrodes and good conduction of the electrical current from the cell, are attached to the electrodes. The anode and cathode contain so-called electrocataylsts, which provide catalytic support to the particular reaction (oxidation of hydrogen and reduction of oxygen respectively). The metals in the platinum group of the periodic system of elements are preferably used as catalytically active components. Mainly, so-called support catalysts are used, in which the catalytically active platinum group metals have been applied in highly dispersed form to the surface of a conductive support material. The average crystallite size of the platinum group metals is between around 1 and 10 nm. Fine-particle carbon blacks have proven to be effective as support materials. The polymer electrolyte membrane consists of proton conducting polymer materials. These materials are also referred to below as ionomers. A tetrafluroethylene-flurovinyl ether copolymer with acid functions, particularly sulfuric acid groups, is preferably used. A material of this type is sold under the trade name Nafion(copyright) by E.I. DuPont, for example. Other ionomer materials, particularly fluorine-free examples such as sulfonated polyether ketones or aryl ketones or polybenzimidazoles, can also be used, however.
For the broad commercial use of PEM fuel cells in motor vehicles, a wide-ranging improvement in the electrochemical cell output and a marked reduction in system costs is required.
An essential precondition for an increase in cell output is an optimum supply and removal of the various reactive gas mixtures to and from the catalytically active centers of the catalyst layers. In addition to the supply of hydrogen to the anode, the ionomer material from which the anode is made must be continuously humidified with water vapour (humidifying water) in order to guarantee an optimum proton conductivity. The water formed at the cathode (reaction water) must be continuously removed to avoid flooding of the pore system in the cathode and hence an obstruction to the supply of oxygen.
U.S. Pat. No. 4,293,396 describes a gas distribution electrode consisting of an open-pore conductive carbon fiber fabric. The pores in the carbon fiber fabric contain a homogeneous mixture of catalyzed carbon particles (carbon particles having catalytically active components deposited thereon) and hydrophobic particles of a binder material.
The German laid-open patent specification DE 195 44 323 A1 presents a gas distribution electrode for polymer electrolyte fuel cells which contains a carbon fiber fabric impregnated with carbon black and polytetrafluoroethylene.
EP 0 869 568 A1 describes a gas distribution layer consisting of a carbon fiber fabric for membrane electrode assemblies. To improve the electrical contact between the catalyst layers of the membrane electrode assemblies and the carbon fiber fabric of the gas distribution layers, the side of the carbon fiber fabric facing the catalyst layer is coated with a microporous layer of carbon black and a fluoropolymer, which is porous and water-repellent as well as being electrically conductive and also has a fairly smooth surface. This microporous layer preferably penetrates no more than half way through the carbon fiber fabric. The carbon fiber fabric can be pre-treated with a mixture of carbon black and a fluoropolymer to improve its water-repellent properties.
WO 97/13287 describes a gas distribution layer (xe2x80x9cintermediate layerxe2x80x9d here) which can be obtained by infiltrating and/or coating one side of a coarse-pore carbon substrate (carbon paper, graphite paper or carbon felt) with a composition consisting of carbon black and a fluoropolymer, which reduces the porosity of part of the carbon substrate close to the surface and/or forms a discrete layer of reduced porosity on the surface of the substrate. The gas distribution layer with this coating is land on top of the catalyst layers of the membrane electrode assemblies. As in EP 0 869 568 A1, one of the objects of the coating is to produce a good electrical contact with the catalyst layers.
Coating the gas distribution layers according to WO 97/13287, U.S. Pat. No. 4,293,396, DE 195 44 323 A1 and EP 0 869 568 with a carbon black/PTFE mixture is a complicated process which has to be followed by drying and calcining at 300 to 400xc2x0 C.
U.S. Pat. No. 6,007,933 describes a fuel cell unit consisting of stacked membrane electrode assemblies and bipolar plates. Flexible gas distribution layers are positioned between the membrane electrode assemblies and the bipolar plates. To supply the membrane electrode assemblies with reactive gases the bipolar plates display gas distribution channels open on one side on the contact surfaces facing the gas distribution layers. In order to improve the electrical contact between the gas distribution layers and the membrane electrode assemblies, the fuel cell unit is assembled under pressure. This involves the risk that the flexible gas distribution layers will penetrate into the gas distribution channels open on one side, thereby obstructing the transport of gas and impairing the electrical output of the fuel cell. This is prevented by means of perforated support plates, for example, which are placed between the gas distribution layers and the bipolar plates. O-ring seals and seals made from PTFE films are used to seal the membrane electrode assemblies.
An object of the present invention is to provide a fuel cell stack exhibiting a simplified construction in comparison to the prior art with the same or improved electrical output.
The above and other objects of the invention can be achieved by means of a PEM fuel cell stack consisting of one or more fuel cells (1) arranged on top of one another, each of which contains a membrane electrode assembly (2) between two electrically conductive bipolar plates (3,4), the surfaces of which are equipped with flow channels (10) open on one side for the supply of reactive gases. The membrane electrode assemblies each comprise a polymer electrolyte membrane (5), both sides of which are in contact with a reaction layer 6,7). The surface area of the reaction layers is smaller than that of the polymer electrolyte membrane. A compressible, coarse-pore gas distribution layer (8,9) made from carbon fiber fabric is inserted between each reaction layer and the adjacent bipolar plates congruent to the reaction layers along with seals (11,12) in the area outside the surface covered by the gas distribution layers, whereby the gas distribution layers in the no-load condition display a thickness D1 and the seals a thickness of D2. The PEM fuel cell stack is characterized in that the gas distribution layers in the PEM fuel cell stack are compressed to 25 to 60% of their original thickness.
According to the invention the cell resistance (resistance of an individual membrane electrode assembly) is reduced by defined compression of the woven gas distribution layers. The aim is preferably to achieve compression of the gas distribution layers to 30 to 50, particularly 35 to 40% of their original thickness D1. Experience has shown that the specific resistance of the carbon fiber fabric can be reduced to below 6 mxcexa9cm by compression. The porosity of the gas distribution layer is also reduced to 20 to 70% of the original porosity, thereby preventing flooding of the pores with reaction water. Both effects lead to a decisive improvement in the electrical output of the fuel cell stack.
The defined compression can easily be set by using seals made from non-compressible material whose thickness D2 is less than the thickness D1 of the compressible gas distribution layers in the no-load condition. On assembling the fuel cell stack the compressible gas distribution layers are compressed to the thickness of the seals, such that the compression of the fuel cell stack is obtained by the ratio D2/D1. In the context of this invention, materials or composite materials whose compressibility is less than 5, preferably less than 1% of the compressibility of the gas distribution layers are designated as non-compressible. Seals made from polytetrafluoroethylene (PTFE), which satisfy the above condition by being reinforced with glass fibers, are preferably used.
The fact that the defined compression of the gas distribution layers avoids the need for the otherwise conventional provision of the gas distribution layers with an electrically conductive microporous layer and the associated complex process steps is particularly advantageous. Moreover, there is also no need for the use of a special support plates intended to prevent penetration by the carbon fiber fabric of the gas distribution layers into the flow channels on the bipolar plates.
The flow channels on the bipolar plates are connected to supply and delivery channels for the reaction gases, which pass vertically through the entire stack of plates outside the area of the membrane electrode assemblies in a peripheral area of the bipolar plates. Between the supply and delivery channels the flow channels are arranged on the contact surfaces of the bipolar plates, generally in the form of right-angled meanders or serpentines.
A particularly advantageous embodiment of the PEM fuel cell stack according to the invention is obtained if the weave direction of the carbon fiber fabric from which the gas distribution layers are made is turned at an angle xcex1 of 20 to 70, preferably 30 to 60, and particularly of 45xc2x0 to the flow channels on the bipolar plates, or if the carbon fiber fabric is woven in a structure such that at least 60% of the fibers exhibit an angle of at least 30xc2x0 to the channel structure of the bipolar plates. In this case a further improvement of the transport properties and water content of the gas distribution layers is obtained, since penetration of the fabric into the gas distribution channels and hence obstruction of the gas flow in the channels is further reduced. The same positive effect is also obtained if the gas distribution channels are arranged in a suitable pattern on the bipolar plates.
The PEM fuel cell stacks according to the invention display good access by the reactive gases to the catalytically active centers of the membrane electrode assemblies, effective humidifying of the ionomer in the catalyst layers and the membrane and straightforward removal of the reaction product water from the cathode side of the membrane electrode assemblies.
Commercial coarse-pore carbon fiber fabrics with porosities of 50 to 95% can be used to produce the gas distribution layers according to the invention. Various base materials, which differ in structure, manufacturing process and properties, are available. Examples of such materials are AvCarb 1071 HCB from Textron Inc. or Panex 30 from Zoltek, Inc.
The commercial, coarse-pore carbon fiber fabrics can be impregnated with hydrophobic polymer before use. Suitable hydrophobic polymers are polyethylene, polypropylene, polytetrafluoroethylene or other organic or inorganic hydrophobic materials. Suspensions of polytetrafluoroethylene or polypropylene are preferably used for impregnation. Depending on the application, the carbon fiber substrates can be loaded with 3 to 30 wt. % of hydrophobic polymer. Loads of 4 to 20 wt. % have proven to be particularly effective. Different loads can be used for the gas distribution layers at the anode and cathode. The impregnated carbon fiber substrates are dried at temperatures of up to 250xc2x0 C. under vigorous air exchange. Drying in a circulating air drying oven at 60 to 220, preferably 80 to 140xc2x0 C., is particularly preferred. This is followed by sintering of the hydrophobic polymer. In the case of PTFE, for example, this takes place at a temperature of 330 to 400xc2x0 C.